The objective of this research program is to further our understanding of how genetic, environmenal, and physiological (primarily hormonal) variables act and interact to produce differences in behavior. The experimental subjects are laboratory mice (Mus musculus). The behavior under study is the masculine sexual response pattern of this species. The current proposal is based primarily on two different genotypes. B6D2F1 male mice retain all elements of sexual behavior for months and even years following castration. CDF1 male mice lose sexual responsiveness rapidly following castration. Castrated males of these strains also respond differently when various amounts of certain hormones are administered. Subjects for the present research are noncastrated males, castrated males, and neonatally androgenized females of both strains. Independent variables include several androgens, estrogens, and progesterone administered in different doses. Dependent variables include a variety of behavioral, morphological, and reproductive measures. Certain of the hormones will also be administered in combination. Special emphasis is placed upon the time required to recover sexual capacity following an ejaculatory reflex and the number of ejaculatory reflexes exhibited in a time-limit test.